440 A. P. COLEMAN 
The coarse-grained dike with its individuals of nepheline 
half a foot wide is not easy to study in thin sections. The 
nepheline proves under the microscope to have been slightly 
fractured, very narrow fissures being filled with a rather brightly 
polarizing mineral, perhaps feldspar. The few inclusions are much 
like those of the nepheline in the schistose rock, but in one sec- 
tion rather large portions of muscovite are enclosed. The large 
crystals of pale lavender muscovite have no unusual characters 
except their often perfect idiomorphy as against nepheline and 
sodalite. The crystals are not hexagonal in cross section but 
four sided having one angle of about 60°. The basal cleavage 
is somewhat inclined to the prismatic edges, though a series of 
pyramids having a very long C axis makes it difficult to deter- 
mine the angle. In thin sections cut across the cleavage this 
muscovite has an extinction angle of 3° to 5°. 
If single thin sections were to be diagnosed alone four quite 
distinct types of rock could be described from this outcrop; a 
nepheline-muscovite rock; a rock made up chiefly of scapolite 
and muscovite with a little biotite, plagioclase, and nepheline; a 
rock containing about equal parts of plagioclase and nepheline 
with some mica; and a rock consisting of orthoclase, microcline 
and nepheline with some mica. There are, however, transitions 
between these varieties, and it’would be unwise to split up what 
is so evidently a geological unit into rocks of different names 
when the whole is so well defined in general character, though 
each hand specimen shows differences from its neighbors. 
No analysis has been made of this rock, but one specimen 
yielded nearly 10 per cent. of corundum in a heavy solution. 
As there was no magnetite nor other heavy mineral present the 
separation was very complete, corundum having a much higher . 
specific gravity than the other ingredients. Since every mineral 
present, except the trifling quantity of calcite and apatite, contains 
alumina, nepheline in particular to the extent of more than 30 
per cent., this oxide must occur in very large amounts. On the 
other hand iron oxides must be very low, since the only iron- 
bearing constituent is biotite. 
